1. Field of the Invention
This invention relates to a process for producing a sputtering target used for the formation of a thin film by sputtering. More specifically, it relates to a method of treating the surface to be sputtered of a sputtering target.
2. Description of the Related Art
The formation of a thin film by sputtering is characterized in that the adhesion of the thin film is strong; the film thickness can easily be controlled; when the thin film is formed from an alloy, the reproducibility of alloy composition is excellent; the thin film can be produced from a material with a high melting temperature. Therefore, the thin film formation by sputtering is widely employed in various uses including, for example, transparent electrically conductive films for liquid crystal displays, recording layers of hard discs and lead metals for semiconductor memories.
The composition of sputtering targets varies depending upon the particular use thereof. For example, sputtering targets are made of chromium, molybdenum, carbon, aluminum, ITO (indium oxide-tin oxide), ZAO (zinc oxide-aluminum oxide) and STO (strontium titanate). In general, sputtering targets are made by various methods wherein a sputtering target-forming material is shaped and sintered, which include, for example, a hot isostatic pressing (HIP) method, a pressureless sintering method or a vacuum induction melting method, followed by machining into a desired size (these shaping and sintering methods are generically referred to as the "shaping-sintering method").
The machining is carried out for obtaining a target of a desired size with high precision and for removing a surface layer stained with impurities. The machining is usually conducted by using a machining center, a surface grinding machine and a milling machine. (A substantially long time is required the operation of machining.)
Among the methods of forming a thin film by sputtering, magnetron sputtering has become increasingly prevalent which is characterized as forming a plasma with a high ionic current density by an interaction between the electric field and the magnetic field in the closed magnetic field formed on a target by an electromagnet or a permanent magnet, attached on the backside of the target. In the magnetron sputtering, film formation can be effected at a high rate under a relatively low gas pressure, but the ionic current density on the target surface is difficult to make uniform, and thus, the target is eroded non-uniformly at sputtering.
To solve the problem of magnetron sputtering, the present inventors have proposed a high-efficiency target in Japanese Unexamined Patent Publication No. 1-290764, which is characterized as having enhanced utilization because the target is thick in an erosion region in which the ionic current density is high and sputtering occurs intensely and is thin in a non-erosion region in which sputtering occurs to the least extent.
The target surface of the high-efficiency target is not planar but has a raised portion thereon. Therefore, after the target is sintered, the surface machining is difficult to conduct, and thus, it is difficult or even impossible to remove impurities from the target surface and further to polish the target surface for providing a smooth surface.